In the foundry art, molds and cores used in making metal castings are prepared from an aggregate such as sand and a curable binder. In recent years, the curable binders of the no-bake type, i.e., those requiring little or no additional heat other than that available at ambient temperatures, have gained wide acceptance in foundry practice. The numerous advantages of these no-bake systems are well known to those skilled in the foundry art. Typically, to be useful as a no-bake binder, the system must have a sufficient bench or work life, must impart a rapid stripping strength to the cores and must develop good tensile strength when completely cured at room temperature. In addition, the binder system should work equally well on all foundry aggregates, be relatively insensitive to moisture and give castings without surface defects with all metal types.
Although phenol-formaldehyde resins have been crosslinked with diisocyanates for many years in the coatings and adhesives industries, it has been only relatively recently that phenol-formaldehyde resins crosslinked with isocyanates have been used in the foundry process for mold and core making.
U.S. Pat. Nos. 3,409,579; 3,432,457; 3,485,797; 3,676,392; 3,702,316; and 3,726,867 teach the use of special phenol-formaldehyde resin which contain benzylic ether linkages (Figure I, portion m) which are prepared at temperatures in excess of 100.degree. C. with special divalent metal ion catalysts. ##STR1##
These special resins are described as high molecular weight phenolic resins containing more dimethylene ether linkages than methylene linkages between the phenol rings, i.e., m is greater than n in Figure I, and the number of repeating aromatic rings is preferably 4 to 10. In addition these resins were described as being essentially anhydrous in that they contain less than 5% and preferably less than 1% water.
The production of phenolic resole mixtures are normally prepared using an alkaline catalyst such as sodium hydroxide with a molar ratio of formaldehyde to phenol greater than one. This reaction can be divided into two temperature ranges; (1) reaction temperatures in excess of 90.degree. C., which yields conventional resole resins containing methylene bridges [Figure II], ##STR2## wherein the average n is typically 2 or more, and (2) reaction temperatures less than 90.degree. C. The latter temperature condition will yield low molecular weight materials where the degree of advancement will depend on the actual temperature used and the length of the reaction. The molecular weight of these low temperature alkaline-catalyzed one-step phenolic mixtures is a function of the degree of reaction. These materials may range from mixtures of mononuclear methylolated phenols of the type shown in Figure III with molecular weights of 125-150, and up to resins with molecular weights of 1,000 or greater, depending upon time of reaction. ##STR3##